1. Field of the Invention
The present invention relates generally to the manufacturing of soot preforms, and particularly to treating a soot preform with a reducing agent.
2. Technical Background
In the manufacturing of optical fibers (hereinafter xe2x80x9cfiberxe2x80x9d) and other products which can be produced from a soot preform, a preform having numerous impurities may cause various defects in the final product. The effect of these defects may be exhibited in terms of attenuation of a signal being propagated along the fiber or a significant reduction in the transmission of a light signal of a certain wavelength through the resultant glass article. In making an optical fiber, it is desired to minimize attenuation and accordingly eliminate the defects that cause the attenuation.
A sign that a fiber includes impurities is that the attenuation the fiber exhibits at a certain wavelength increases the longer the fiber is in operation. This effect of increasing attenuation is most noticeable at wavelengths of at least 1200 nm.
Previous attempts to minimize the potential for a fiber to undergo the above attenuation increases include drying the preform with chlorine gas. Traditionally, the preform is placed in a furnace prior to consolidation. The furnace is charged with a helium gas stream which includes approximately two percent (2%) chlorine (Cl2) and the furnace is heated to a temperature of approximately 1000xc2x0 C., for up to about two (2) hours. However, treating a soot preform with chlorine gas has not satisfactorily reduced the aforementioned increased attenuation exhibited by the fiber.
Another limitation of the chlorine treatment of a soot preform is that it is limited to glass compositions that are relatively inert to chlorine treatments. Examples of glass compositions that are relatively inert to a chlorine treatment include silica glass or germanium doped silica glass. However, not all components of a silicate glass composition may be inert to the aforementioned chlorine treatment. Examples of such silicate glass compositions that are not inert to the aforementioned chlorine treatment are those compositions that contain alumina, antimony, alkali oxides, boron oxides, phosphorus oxides, or alkaline-earth oxides. The aforementioned chlorine treatment has been found to strip off the alumina, antimony, alkali, or alkaline earth containing compounds from the preform.
The aforementioned chlorine treatment has further proven inappropriate for some glass products that are fluorinated after the chlorine treatment for the reason that the preform will retain some of the chlorine from the chlorine treatment even after the preform is fluorine doped. The retention of chlorine will affect the optical properties of the final glass product. One example of a glass product in which the inclusion of chlorine is not desired is a lithography photomask plate. For example, the presence of chlorine in lithography photomask plates reduces transmission at various wavelengths of interest. One such wavelength is 157 nm. It is has been demonstrated that the presence of only about 75 ppm of chlorine in the photomask plate will reduce transmission by at least about 50% of a light having a wavelength of about 157 nm. For at least the above reasons, there exists a need for new methods to treat a soot preform.
Additionally, fluorine doped optical fibers have exhibited attenuation increases at several wavelengths. The attenuation spectrum of a fluorine doped optical fiber will exhibit absorption peaks in various wavelengths, such as, 1440 nm, 1546 nm, 1583 nm, and 1610 nm. For at least the reason that the 1583 nm wavelength is in L band transmission window, there is a need to make an optical fiber with a fluorine doped region that does not exhibit an absorption peak at a wavelength of 1583 nm or any of the other cited wavelengths.
The present invention relates to a soot preform that can be used to make optical products and to treating the soot preform. One embodiment of the present invention is a method of treating the soot preform. The method includes exposing a soot preform, in a furnace, to a substantially chlorine free atmosphere comprising at least one reducing agent to consume excess oxygen present in the furnace. A preferred reducing agent is carbon monoxide.
An advantage of practicing the above method of the invention is that it may be used to consolidate a soot preform in an environment that is not an overtly oxidizing environment. The method of the invention may also be practiced to remove excess oxygen from the preform and produce an optical fiber with better long term attenuation properties. A further advantage that will result from practicing the above method is that the fiber draw speed can be increased. One particular excellent application of the invention is including the above described method in a process for the production of a non-zero dispersion shifted optical fiber.
In a second method for practicing the invention, the preform is heated to a temperature of less than about 1000xc2x0 C. and the preform is exposed to a reducing atmosphere. A preferred reducing atmosphere comprises carbon monoxide.
One advantage of practicing the above embodiment of the invention is that a chlorine free glass may be formed from a preform made in accordance with the above embodiment. A chlorine free glass means at least a preform that has not been exposed to chlorine during manufacturing. A chlorine free glass has an excellent application as a lithography photomask plate, in that a chlorine free photomask can satisfactorily transmit light at a wavelength of less than about 160 nm. Another advantage of practicing the aforementioned embodiment of the invention is that it may be used to treat a soot preform that includes alumina, antimony, an alkali oxide, an alkaline earth oxide, or any other compound that would react with chlorine and volatilize off or are transformed into crystalline chlorides. A further advantage of the above embodiment of the invention is that it may be practiced at low temperatures, temperatures below about 1000xc2x0 C.
A third method of practicing the invention is a method of making an optical fiber preform. The method includes the step of doping a soot body in an atmosphere comprising carbon monoxide and at least one fluorine containing compound have the general formula CnF2n+2. Preferably xe2x80x9cnxe2x80x9d is a positive whole number. One advantage of incorporating the aforementioned method of the invention into a process for making an optical fiber with a fluorine containing region is that the fiber does not exhibit an absorption peak at wavelengths of about 1440 nm, about 1546 nm, about 1583 nm, or about 1610 nm.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.